Move all ufc detection, version, etc to ffc.backends.ufc

[Prof Garth Wells]

UFC config functions are spread across ffc and ffc.backend.ufc. Would be tidy to keep it all under ffc.backend.ufc.

[Martin Sandve Alnæs]

I've done this plus a signature bugfix in a branch where I needed the bugfix.

[Martin Sandve Alnæs]

Do we need to keep ffc.ufc_config for compatibility or is it OK to remove it? Is only dolfin using it?

[Prof Garth Wells]

Since only DOLFIN uses UFC, I presume it's fine to remove.